Increasingly smaller and faster semiconductor circuitry has fueled an information
technology boom over the past four decades, producing cheaper and more powerful
computing devices that have boosted virtually every aspect of our economy. But
fundamental limits imposed by the laws of physics threaten to halt continued
miniaturization, clouding the future of silicon-based semiconductors.

A paper published in the September 14 issue of the journal Science provides some
good news: though significant challenges lie ahead, the semiconductor industry has
the potential for at least two more decades of continuing miniaturization. That
opportunity should encourage the research necessary to master nanometer-scale
technologies needed to overcome these challenges, the paper’s author contends.

“The laws of physics reveal the potential for 20 more years of exponential progress
ahead of us,” said James D. Meindl, professor of electrical and computer engineering
and director of the Microelectronics Research Center at the Georgia Institute of
Technology. “If the engineers are clever enough – which historically they have been –
they will be able to find ways to produce the nanoelectronic structures that physics
says are feasible and reasonable.”

Based on a comprehensive analysis of the fundamental, material, device, circuit and
system limits on silicon semiconductors, Meindl predicts engineers will be able to
downsize transistors by a additional factor of ten, producing terascale integration
chips containing more than a trillion transistors. (Chips poised for production today
contain a billion transistors).

Understanding the fundamental limits governing future miniaturization should give
semiconductor companies the confidence to pursue costly nanotechnology
innovations necessary to produce the trillion-transistor chips. The Georgia Tech paper
is the first to provide a comprehensive look at those limits.

“It is reassuring to know that you are not fighting against a law of physics,” Meindl
said. “Knowing the fundamental limits gives you hope that cleverness can produce the
inventions that you need to continue miniaturization. Now that the fundamental limits
have been pinned down, we can start to see what other factors will impede us as we
approach this limit.”

The semiconductor industry publishes an annual “roadmap” that lays out the
challenges expected for the next 15 years. White blocks represent proven solutions,
yellow blocks show where promising technologies exist, and red blocks define
challenges without solutions. The term “red brick wall” describes portions of the map
containing large numbers of red squares – and hence the greatest challenges.

“The red brick wall for the industry is now pretty serious, even five years from now,”
Meindl said. “But that has been the case generally for the industry, which has always
needed clever inventions that couldn’t be predicted. In order to keep this technology
going, we’ll have to turn those red squares to yellow and white.”

To produce trillion-transistor chips, he noted, the industry must be able to
economically mass-produce structures on the nanometer-size scale. That means
double-gate metal-oxide-semiconductor field effect transistors (MOSFETs) with gate
oxide thicknesses of about one nanometer, silicon channel thicknesses of about
three nanometers and channel lengths of about 10 nanometers – along with
nanoscale wires for interconnecting such tiny components.

The fundamental limit defines the minimum amount of energy needed to perform the
most basic computing operation: binary logic switching that changes a 0 to a 1, or
vice-versa. Meindl and collaborators Jeffrey A. Davis and Qiang Chen found that the
fundamental limit depends on just one variable: the absolute temperature. Based on
this fundamental limit, they studied a hierarchy of limits that are much less absolute
because they depend on assumptions about the operation of devices, circuits and

The researchers studied the fundamental limit from two different perspectives: the
minimum energy required to produce a binary transition that can be distinguished,
and the minimum energy necessary for sending the resulting signal along a
communications channel. The result was the same in both cases.

The fundamental limit, expressed as E(min) = (ln2)kT, was first reported 50 years ago
by electrical engineer John von Neumann, who never provided an explanation for its
derivation. (In this equation, T represents absolute temperature, k is Boltzmann’s
constant, and ln2 is the natural log of 2).

Though this fundamental limit provides the theoretical stopping point for electrical and
computer engineers, Meindl says no device will ever operate close to it because
designers will first bump into the higher-level limits. For example, electronic signals
can move through interconnects no faster than the speed of light. And quantum
mechanical theory sets minimum size restrictions on devices.

Though the limits provide a final barrier to innovation, Meindl believes economic
realities will bring about the real end to advances in silicon microelectronics.

“What has enabled the computer revolution so far is that the cost per function has
continued to decrease,” he said. “It is likely that after a certain point, we will not be able
to continue to increase productivity. We may no longer be able to see investment pay
off in reduced cost per function. Because the stakes are getting so high in terms of
factories needed to turn out these denser and denser chips, the number of
companies that can afford the multi-billion dollar factories has been dwindling.”

The future of silicon semiconductors ultimately depends on nanotechnology.

“What happens next is what nanotechnology research is trying to answer,” Meindl
said. “Work that is going on in nanotechnology today is trying to create a discontinuity
and jump to a brand new science and technology base. Fundamental physical limits
encourage the hypothesis that silicon technology provides a singular opportunity for
exploration of nanoelectronics.”


The research has been sponsored by the Defense Advanced Research Projects
Agency under Contract F33615-97-C1132, the Semiconductor Research Corporation
under Contract HJ-374 and the Georgia Institute of Technology.

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